Power amplifiers usually used in radio transmitters for broadcast, cellular, and satellite systems, are indispensable components that have to be efficient and linear, in addition to being able to simultaneously amplify many radio channels (frequencies) or independent user data channels, spread across a fairly wide bandwidth. A power amplifier, such as a radio frequency (RF) power amplifier, also has to perform amplifications efficiently in order to reduce power consumption and to increase its longevity. High linearity is required because a non-linear input-output signal characteristic of a power amplifier often results in a broadened spectrum around the desired amplified signal, and an unwanted in-band component of the signal, which lead to bad system performance especially in multicarrier telecommunications systems (e.g. WCDMA) which are known to be particularly sensitive to the effects of non-linearities.
To decrease the effects of non-linearity, several linearization schemes could be used. One such linearization scheme is known as feed-forward, where a signal is injected after the amplifier that cancels the non-idealities. Another linearization scheme usually used is to predistort (modify) the signal at the input of the amplifier in order to give an undistorted amplified signal at the output of the amplifier. This technique is called predistortion.
An additional important key factor of RF power amplifiers used in multicarrier telecommunications systems (e.g. WCDMA) is as mentioned above, the amplifier efficiency.
The amplifier efficiency must be kept high in order to reduce the need for cooling as well as the overall power consumption, and to increase the lifetime of the power amplifier. Conventional power amplifiers have low efficiency especially when transmitting signals with a large peak-to-average power ratio. As an example, a wideband signal generally has an average power (amplitude) that is normally much smaller than the peak power (amplitude) and since a conventional linear RF power amplifier generally has an efficiency that is proportional to its output amplitude, its average efficiency is consequently very low for such signals having a large peak-to-average power ratio.
In response to the low efficiency of conventional linear power amplifiers when transmitting signals with a large peak-to-average power ratio, two methods or two amplifier structures have been widely utilized. The Doherty amplifier (or the Doherty amplification method), is described in W. H. Doherty, “A new high efficiency power amplifier for modulated waves,” Proc. IRE, vol. 24, no. 9, pp. 1163-1182, September 1936, and the Chireix outphasing system (or the Chireix amplification method) is described in H. Chireix, “High power outphasing modulation”, Proc. IRE, vol. 23, no. 11, pp. 1370-392, November 1935.
The Doherty amplifier uses one non-linear and one linear amplifier. A first power amplifier is driven as a linear amplifier in class B, and a second power amplifier having non-linear output current modulates the impedance seen by the first amplifier, through an impedance-inverting quarter wave line. Since the non-linear output current of the second power amplifier is zero below a certain output voltage, the second power amplifier does not contribute to power loss below this voltage.
The Doherty amplifier's output power level back-off where the efficiency reaches a maximum in the efficiency curve of the Doherty amplifier is at half the maximum output voltage. The location of the output power level back-off can be changed by changing the impedance of the quarter-wave transmission line (or transformation (matching) network). Thus the size of the transformation (matching) dictates the location of the lower efficiency maximum of the Doherty power amplifier. Even though the Doherty amplifier can be extended to three or more amplifiers in order to obtain more maximum points on the efficiency curve, this usually leads to a requirement for very unevenly sized amplifiers i.e. transistors.
The principal of the Chireix outphasing system is to create amplitude modulation using two amplifiers operating at constant amplitude together with a special type of combining network. By altering the differential phase-shift between the two amplifiers, amplitude modulation is created. The combination of generally two phase modulated constant amplitude signals thus enables amplitude modulation. After up-conversion and amplification through RF chains (e.g. mixers, filters and amplifiers), the signals are combined to form an amplified signal in an output combiner network. The phases of these constant amplitude signals are chosen so that the result from their vector-summation yields the desired amplitude. The compensation reactances, denoted +jX and −jX respectively, in the output network of the Chireix amplifier, are used to extend the region of high efficiency to include lower output power levels. The efficiency of Chireix systems is derived in R. F. Raab, “Efficiency of outphasing RF power amplifier systems”, IEEE Trans. Communications, vol. COM-33, no. 10, pp. 1094-1099. October 5.
An advantage with the Chireix amplifier is the ability to change the efficiency curve to suit different peak-to-average ratios. The peak output power is equally divided between the amplifiers irrespective of this adjustment, which means that equal size amplifiers can be used. The change of the efficiency curve may be performed by changing (tuning) the size of the reactances (X) in order to tune a combining network of a Chireix amplifier, thus achieving peak efficiency at an average output power. This approach is proposed in M. El-Asmar, A. B. Kouki “Improving Chireix Combiner Efficiency Using MEMS Switches”, IEEE CCECE/CCGEI, Ottawa, pages 2310-2313, May 2006.
In the above mentioned prior art publication, the length of tuning stubs that are used to create the compensation reactances for the Chireix combiner, is varied. The response time of MEMS (Micro Electro Mechanical System) switches are used to connect and disconnect the different stubs (usually two) at the input of the combiner. This exchange of the two stubs occurs at a fixed level of the phase between the two input signals in order to increase the efficiency of the Chireix power amplifier. MEMS-switches are however mechanical devices which means reliability issues over time. Furthermore, the switches available today are commonly quite small and thus will be severely affected by the amount of power passed through the combiner network from each amplifier of the Chireix amplifier. In addition, finite switch time may cause additional problems by introducing “jumps” by the load seen from each amplifier thus affecting the efficiency of the amplifier.
In general, the Chireix and Doherty amplifiers have efficiency maxima at some fixed medium output levels. This is considered optimal for some fixed signal amplitude distribution but less than optimal for all other. This is due to that the efficiency getting lower moving away from these signal envelope amplitudes.
In the U.S. Pat. No. 7,221,219, a composite power amplifier structure is suggested which essentially is based on a combination of the auxiliary amplifier of a Doherty amplifier and at least one pair of amplifiers forming a Chireix pair. The Doherty part of the composite amplifier is driven in the same manner as the auxiliary amplifier of a Doherty amplifier. Each Chireix pair is driven by drive signals having amplitude dependent phase over at least a part of the dynamic range of the composite amplifier. In this prior art document, the efficiency of the composite amplifier is improved by letting the different pairs have amplitude dependent phase in different part of the dynamic range of the composite amplifier.
Another method of improving the average efficiency of a RF power amplifier is by dynamically adjusting a matching network of the RF power amplifier. However, the dynamic matching components of the matching network may be slow, may lose efficiency above those of fixed components and/or may require significant power in order to perform the adjustment since the power is generally proportional to the bandwidth of the adjustment/tuning process.